Magnetic inspection techniques provide an interesting alternative to more traditional non-destructive evaluation techniques of ultrasonics, eddy currents and radiography. They are of interest because of their perceived sensitivity to both stress and microstructure of the material. The possibility of usefully employing magnetic property measurements for materials evaluation has been known in principle for many years. However, although it was possible to demonstrate significant changes in the magnetic properties of materials as a result of thermal and mechanical treatment, the changes proved difficult to interpret because of their apparent complexity. For example, a given specimen subjected to the same external field, when also subjected to identical stress cycles, could exhibit changes in magnetization which were opposite in sign. Measurements could thus be made within a few minutes of each other with no apparent change in the external condition, but significantly different measurement results.
The Barkhausen effect is a magnetic effect, and Barkhausen effect evaluation fits within the broader category of magnetic inspection methods. The Barkhausen effect is postulated on small magnetic domains grouped together to form a larger magnetic sample. The domains are randomly distributed and positioned when the specimen is in a non-magnetic state. When a magnetic flux is applied to the material, the flux forces reorientation of the domains, and the domains are observed to shift suddenly. Shifting and change in domain size occur suddenly, creating magnetic responses, and the shifts occur at various depths in the material.
The Barkhausen effect can generate a relatively complex response characteristic. While researchers have studied the characteristic, the characteristic is so complex that it has not been possible heretofore to adequately analyze it in an efficient manner to derive very detailed properties on the characteristics of the sample which produced the result.
For example, it has been typical to detect the Barkhausen response then merely rectify or average it to produce a "Barkhausen number" which can then be compared to similar numbers for other samples. Other characteristics of the Barkhausen response may have been studied from time to time, but insofar as applicants are aware, means have not been available to accurately and reliably correlate information relating to the exciting or perturbing signal and multiple characteristics of the Barkhausen response. It has now been found that the ability to derive and correlate such information for a given excitation will allow the generation of significant detail on the structure of a magnetic sample at or near its surface, detail much more precise than has been available heretofore.
It has been learned that ferromagnetic materials exhibit hysteresis in the dependence of magnetization M on magnetic field H. As a result, the state of the system cannot be uniquely defined simply by external factors such as field strength and stress. A complete description of the system must include the prevailing magnetization and its history, and that information is not a single valued function of H and .sigma.. However, insofar as applicants are aware, a system has not been available for controllably altering the field applied to a specimen whose surface characteristics are to be studied, for deriving adequate information including both frequency spectrum and amplitude information from the Barkhausen response in order to more completely utilize the Barkhausen response of the material to deduce material characteristics, and also to understand that response by virtue of knowledge of the fields and stresses which have been applied prior to the instant of measurement.